The application of coatings to metals to inhibit corrosion is well established. This is particularly true in the area of packaging containers such as metal food, beverage containers and twist off caps. Coatings are typically applied to the interior of such containers to prevent the contents from contacting the metal of the container. Contact between the metal and the packaged product can lead to corrosion of the metal container, which can contaminate the packaged product. This is particularly true when the contents of the container are chemically aggressive in nature. Protective coatings are also applied to the interior of food, beverage containers and closures such as twist off caps to prevent corrosion in the headspace of the container between the fill line of the food product and the container lid.
Packaging coatings should preferably be capable of high-speed application to the substrate and provide the necessary properties when hardened to perform in this demanding end use. For example, the coating should be safe for food contact, not adversely affect the taste of the packaged food or beverage product, have excellent adhesion to the substrate, resist staining and other coating defects such as “blushing” and/or “blistering,” and resist degradation over long periods of time, even when exposed to harsh environments. In addition, the coating should generally be capable of maintaining suitable film integrity during container fabrication and be capable of withstanding the processing conditions that the container may be subjected to during product packaging.
Conventional closures for packaging containers incorporate one or more coatings that are typically derived from physical and/or chemically curable formulations that include one or more thermoplastic and/or thermosetting resins. (e.g., vinyl chloride polymers such as, for example, poly(vinyl chloride) (“PVC”)), or epoxy-derived resins).
When these coatings are applied to a substrate and cured, the coatings and/or the coated substrate can degrade and/or discolor. For example, the degradation products from PVC-based coatings, such as hydrochloric acid (“HCl”), can attack the substrate. Additionally, these degradation products may cause yellowing of white closures. To stabilize thermoplastic materials such as PVC and reduce degradation, epoxy resins and/or other stabilizers typically are added to the coating formulations. It has been a common practice to use epoxy novolacs and epoxidized linseed oil. These epoxy resins typically incorporate polyglycidyl ethers of aromatic polyols such as bisphenol A diglycidyl ether (often referred to as “BADGE”).
Coatings derived from epoxy functional materials may include small amounts of (i) unreacted bisphenol A (“BPA”) or the epoxidized ethers of aromatic polyols such as BADGE; and/or (ii) low-molecular-weight components containing BPA or BADGE. In the food packaging industry, these materials potentially can migrate into packaged foodstuffs over time. In addition, conventional coating systems for use in packaging applications that require exposure to aggressive or corrosive food or beverage products often use a BPA- or BADGE-containing epoxy-phenolic size coat in combination with a topcoat formulation containing thermoplastic materials such as PVC.
Although the balance of scientific evidence available to date does not indicate clearly that traces of the aforementioned compounds pose health risks to humans, these compounds are perceived by some as being potentially harmful to human health. Consequently, there is a desire to reduce or eliminate these compounds from food-contact coatings. However, it has been problematic to formulate coating formulations without these materials, or with these materials that exhibit very low or non-detectable levels of mobile forms of these compounds, while still retaining desired coating characteristics (e.g., flexibility, adhesion, corrosion resistance, stability, abrasion resistance, etc.).
Many commercially available coatings systems are based on a two-layer system. A first layer functions as a base (also referred to as a primer or size layer) applied to the substrate. A second layer is provided on the base layer to function as a top coat. The combination of layers provides levels of performance (e.g, adhesion, blush resistance, stain resistance, and corrosion resistance) that cannot be achieved by either layer alone.
Other commercially available coating systems are based on a single layer, or monocoat system. Monocoat systems offer the potential to be substantially more economical and efficient to manufacture. A monocoat involves a single composition that is applied as a single layer to the substrate and cured to form the monocoat. Consequently, the single coating must demonstrate the desired levels of performance (e.g, adhesion, blush resistance, stain resistance, and corrosion resistance) on its own without contribution from other coating formulations. It is challenging to develop monocoat systems that can demonstrate so many desired levels of performance. This is one reason why more expensive multilayer coating systems still dominate the market for closure coatings.
Monocoat systems that incorporate PVC resins and epoxy functional phenolic resins derived from BPA generally are viewed as setting the performance standard by which monocoat systems are judged. It is desirable to formulate monocoats without PVC and BPA content. However, formulating such alternative monocoat systems that match or approach the performance of the PVC/BPA systems has proven to be a significant challenge. As a result, there is a continuing and strong need for improved, alternative coating compositions that can be used in monocoat systems, particularly in the packaging industry for food and beverages.